Electronic structures of p-benzoquinone and cyclohexane-1,4-dione radical cations. Electron spin resonance study

Abstract

Exposure of dilute solutions of p-benzoquinone and cyclohexane-1,4-dione to ⁶⁰Co γ-rays at 77 K gave the corresponding cations, identified by their e.s.r. spectra. These both comprised asymmetric quintets indicating coupling to four equivalent protons and slight g-value variation. The ¹H coupling constants of ca. 18 and 13.5 G, respectively, show that spin delocalisation into the 4 C—H units is remarkably similar for these cations. For both cations the experimental results suggest a SOMO comprising the two in-plane oxygen p orbitals with delocalisation through the σ-framework.

A similar cation was obtained from 2,6-dimethylbenzoquinone, but the tetramethyl derivative apparently failed to give a cation of this structure. Possible explanations for this are considered.